A recent study has confirmed what scientists have long suspected: heavy marijuana use results in reduced release of dopamine in the brain; a phenomenon that has until now been associated with other kinds of drug addiction. As part of the new research, a team has found evidence of impaired dopamine system among heavy cannabis users. Speaking about the project, Anissa Abi-Dargham, a professor at the Columbia University Medical Center (CUMC) and the paper’s lead author, said:

In light of the more widespread acceptance and use of marijuana, especially by young people, we believe it is important to look more closely at the potentially addictive effects of cannabis on key regions of the brain.

It has long been known that excessive and prolonged use of drugs of abuse, including cocaine and heroin, leads to lower release of the neurotransmitter in striatum, a part of the brain involved in memory, attention and impulsive behavior. According to the scientists at Columbia University, heavy cannabis usage seems to have similar effects on the synthesis and release of dopamine.

Image Credit: Wendy McCormick

For the research, recently published in Nature’s Molecular Psychiatry journal, the team worked with two groups, one comprising of 11 marijuana addicts aged 21 to 40, and the other made up of 12 healthy controls. As the researchers point out, the average member of the former group started using cannabis at the age of 16, became dependent by 20, and has remained severely dependent on it for the last 7 years. A month before the study, the members of the first group were asked to smoke daily.

With the help of positron emission tomography (PET), the team measured the amount of dopamine released in the striatum region of the brain, as well as the surrounding parts, such as thalamus, globus pallidus and midbrain. To ensure that the PET scans were not showing the acute effects of the drug’s intake, the cannabis users were kept in the hospital for seven days of abstinence.

When compared with controls, the cannabis users exhibited substantially lower dopamine release in the brain, specifically in the striatum, the globus pallidus and the regions responsible for associative ans sensorimotor  learning. The members of both groups were made to perform a number of working memory tasks to determine the effects reduced dopamine availability has on learning and other cognitive functions.

While there was no marked difference in the task performance of the two groups, the researchers found that participants with lower dopamine release performed poorly in all the tasks. Dr. Abi-Dargham added:

We don’t know whether decreased dopamine was a preexisting condition or the result of heavy cannabis use. But the bottom line is that long-term, heavy cannabis use may impair the dopaminergic system, which could have a variety of negative effects on learning and behavior.

Source: Columbia University